🥸Advanced Computer Architecture Unit 5 – Branch Prediction & Speculative Execution

What's the Big Deal?

• Branch prediction and speculative execution are crucial techniques employed in modern processors to improve performance and efficiency
• Enable processors to make educated guesses about the outcome of branch instructions before they are actually executed
• Allow the processor to speculatively execute instructions along the predicted path while waiting for the branch outcome to be determined
• Techniques take advantage of the inherent parallelism and pipelining capabilities of modern processors
• Significantly reduce the impact of control hazards and branch penalties on processor performance
• Without branch prediction and speculative execution, processors would frequently stall waiting for branch outcomes leading to suboptimal utilization of resources
• Techniques have become indispensable in achieving high-performance computing and meeting the demands of complex software applications

Key Concepts

• Branch instructions are control flow instructions that determine the next instruction to be executed based on a condition
• Branch prediction involves predicting the outcome of a branch instruction before it is actually evaluated
• Branch predictors are hardware components that implement branch prediction algorithms and maintain prediction history
• Speculative execution refers to the execution of instructions along the predicted path before the branch outcome is known
• Branch target buffer (BTB) is a cache-like structure that stores the predicted target addresses of recently executed branch instructions
• Branch history table (BHT) keeps track of the past behavior of branch instructions to make more accurate predictions
• Pipeline flushing occurs when a branch prediction is incorrect and the speculatively executed instructions need to be discarded
• Branch misprediction penalty is the number of cycles wasted due to an incorrect branch prediction

How It Works

• When a branch instruction is encountered, the branch predictor predicts the likely outcome (taken or not taken) based on historical information
• The processor speculatively executes instructions along the predicted path assuming the prediction is correct
  • Speculative execution allows the processor to continue executing instructions without waiting for the actual branch outcome
  • Results of speculatively executed instructions are stored in temporary registers or buffers
• In parallel, the branch condition is evaluated to determine the actual outcome of the branch
• If the branch prediction is correct:
  • The speculatively executed instructions are committed, and their results become permanent
  • The processor continues execution along the predicted path
• If the branch prediction is incorrect:
  • The speculatively executed instructions are discarded, and their results are ignored
  • The processor flushes the pipeline to remove the effects of the incorrectly executed instructions
  • Execution resumes from the correct branch target address

Types of Branch Predictors

• Static branch predictors make predictions based on fixed rules or heuristics
  • Always taken or always not taken predictors predict branches in a fixed direction
  • Backward taken, forward not taken (BTFNT) predictor predicts backward branches as taken and forward branches as not taken
• Dynamic branch predictors adapt their predictions based on the runtime behavior of branches
  • One-bit predictor uses a single bit to predict the direction of a branch based on its last outcome
  • Two-bit predictor uses a saturating counter to provide hysteresis and improve prediction accuracy
  • Correlation-based predictors (e.g., gshare, gselect) use global branch history to capture correlations between branches
• Hybrid branch predictors combine multiple prediction mechanisms to leverage their strengths
  • Selector-based hybrid predictors choose among different predictors based on their individual performance
  • Tournament predictors use a meta-predictor to select between two or more component predictors
• Neural branch predictors employ neural networks to learn complex branch patterns and make predictions

Speculative Execution Explained

• Speculative execution is a technique that allows the processor to execute instructions before knowing if they are needed
• Processor predicts the outcome of a branch instruction and starts executing instructions along the predicted path
• Speculative execution is performed in parallel with the evaluation of the branch condition
• If the prediction is correct, the speculatively executed instructions are committed, and their results become architecturally visible
• If the prediction is incorrect, the speculatively executed instructions are discarded, and the processor rolls back to the correct execution path
• Speculative execution exploits instruction-level parallelism (ILP) by allowing the processor to execute multiple instructions concurrently
• Enables the processor to hide latencies associated with branch resolution and keep the pipeline full
• Speculative execution is transparent to the programmer and handled entirely by the processor hardware

Performance Impact

• Branch prediction and speculative execution significantly improve processor performance by reducing pipeline stalls and increasing instruction throughput
• Accurate branch predictions minimize the number of pipeline flushes and wasted cycles due to branch mispredictions
• Speculative execution allows the processor to overlap the execution of multiple instructions and hide branch resolution latencies
• Performance gains are highly dependent on the accuracy of branch predictions and the effectiveness of the branch predictor
• Branch-intensive code with hard-to-predict branches may experience lower performance improvements compared to code with more predictable branches
• Mispredicted branches incur a performance penalty as the processor needs to discard speculatively executed instructions and restart execution from the correct path
• Branch prediction and speculative execution have become critical techniques in modern processors to achieve high instructions per cycle (IPC) and overall performance

Challenges and Limitations

• Branch prediction accuracy is limited by the inherent unpredictability of certain branches (e.g., data-dependent branches)
• Complex branch patterns and correlations may be difficult to capture accurately with simple branch predictors
• Large branch prediction tables consume significant chip area and power, leading to design trade-offs
• Deeply speculative execution can lead to increased power consumption and heat dissipation
• Speculatively executed instructions may cause undesired side effects (e.g., cache pollution) if not managed properly
• Speculative execution can introduce security vulnerabilities (e.g., Spectre, Meltdown) by allowing unauthorized access to sensitive data
• Mitigating speculative execution vulnerabilities often involves performance trade-offs and requires careful hardware and software co-design
• Branch prediction and speculative execution may not provide significant benefits for certain workloads with inherently unpredictable or irregular control flow

Real-World Applications

• Branch prediction and speculative execution are ubiquitous in modern high-performance processors (e.g., Intel Core, AMD Ryzen)
• Widely used in desktop, laptop, and server processors to improve performance across a wide range of applications
• Particularly beneficial for compute-intensive workloads (e.g., scientific simulations, data analytics) that exhibit high levels of instruction-level parallelism
• Employed in processors used in gaming consoles (e.g., PlayStation, Xbox) to deliver smooth and responsive gameplay
• Utilized in mobile processors (e.g., ARM Cortex-A series) to provide high performance within power and thermal constraints
• Implemented in processors used in embedded systems and IoT devices to optimize performance and energy efficiency
• Crucial for enabling high-performance computing in data centers and cloud environments where performance and throughput are critical
• Exploited by compilers and runtime systems to optimize code execution and take advantage of the underlying hardware capabilities